I need a one piece competitive swim suit in black..I’m not sure what my size it and it’s limited over here to actually try it on in stores?

Since it’s not summer.I enjoy the speedo brand but I don’t know my size..I’m normally a small/med. in shirts and about a 4 in pants but I’m not sure about the suits. I’ve tried and 8 it was small..I tried a 10 and it was big..I’ve been searching because I was supposed to have one a few days again and I need to get it ASAP How do I find my size without going to the store because their are none out here that have suits anymore..HELP.The more info the better